Radiation therapy system and method

ABSTRACT

The present disclosure relates to a therapeutic apparatus including an MRI apparatus configured to acquire MRI data with respect to a region of interest. The MRI apparatus may include a plurality of main magnetic field coils coaxially arranged along an axis. The MRI apparatus may also include a plurality of shielding coils arranged coaxially along the axis. A current within at least one of the shielding coils may be in the same direction with a current within the main magnetic field coils.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/820,770, filed on Mar. 17, 2020 which is a continuation ofInternational Application No. PCT/CN2019/074540, filed on Feb. 2, 2019,the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure generally relates to a radiation therapy system,and more particularly, relates to an image-guided radiation therapysystem which combines radiation therapy and magnetic resonance imagingtechnique.

BACKGROUND

Radiation therapy on a tumor is currently affected by difficulties totrack the variation (e.g., motion) of the tumor in different treatmentsessions. Nowadays, various imaging techniques may be applied to provideimages of the tumor before or within each treatment session. Forexample, a magnetic resonance imaging (MRI) apparatus may be used incombination with a radiation therapy apparatus to provide MRI images ofthe tumor. The combination of the MRI apparatus and the radiationtherapy apparatus, which forms a therapeutic apparatus, may encounterdifficulties in arranging components of the MRI apparatus (e.g., aplurality of main magnetic field coils, a plurality of magneticshielding coils) and components of the radiation therapy apparatus(e.g., a linear accelerator) in a relatively compact space withoutcausing interferences. For example, passive shielding technics for theradiation therapy apparatus such as providing a shielding structuresurrounding the linear accelerator of the radiation therapy apparatusmay have poor effect for a high intensity (e.g., 1.5 T) of the mainmagnetic field coils. Therefore, it may be desirable to provide atherapeutic apparatus that provides high therapeutic quality and alsohas a compact structure as well.

SUMMARY

According to one aspect of the present disclosure, a therapeuticapparatus including a magnetic resonance imaging (MRI) apparatusconfigured to acquire MRI data with respect to a region of interest(ROI) is provided. The MRI apparatus may include a main magnet bodyincluding a plurality of main magnetic field coils coaxially arrangedalong an axis. The MRI apparatus may also include a plurality ofshielding coils including a first shielding coil, a second shieldingcoil and a shielding coil group arranged coaxially along the axis. Theshielding coil group may be located between the first shielding coil anda second shielding coil.

In some embodiments, the shielding coil group may include a first coilgroup and a second coil group arranged coaxially along the axis.

In some embodiments, the first coil group or the second coil group mayinclude a first coil and a second coil.

In some embodiments, the first coil and the second coil may be arrangedconcentrically.

In some embodiments, a direction of a current within the first coil maybe opposite to a direction of a current within the second coil.

In some embodiments, a radius of the first coil or the second coil maybe larger than that of the plurality of main magnetic field coils.

In some embodiments, a radius of the first coil may be greater than aradius of the second coil.

In some embodiments, the apparatus may further include an annularcryostat. The annular cryostat may include at least one outer wall andat least one inner wall coaxial around the axis, and an annular recessbetween the at least one outer wall and the at least one inner wall. Theannular recess may have an opening formed at the at least one outerwall.

In some embodiments, the annular recess may be located coaxially betweenthe first coil group and the second coil group.

In some embodiments, at least portion of the annular recess may belocated radially between the first coil and the second coil.

In some embodiments, the system may further include a radiation therapyapparatus configured to apply therapeutic radiation to at least oneportion of the ROI. The radiation therapy apparatus may include a linearaccelerator configured to accelerate electrons in an electron beam toproduce a photon beam of the therapeutic radiation. The linearaccelerator may be at least partially located within the annular recessof the annular cryostat. The radiation therapy apparatus may alsoinclude one or more collimation components configured to shape thephoton beam of the therapeutic radiation.

In some embodiments, at least portion of the radiation therapy apparatusmay be located coaxially between the first coil group and the secondcoil group.

In some embodiments, at least portion of the radiation therapy apparatusmay be located radially between the first coil and the second coil.

According to another aspect of the present disclosure, a magneticresonance imaging (MRI) apparatus configured to acquire MRI data withrespect to a region of interest (ROI) is provided. The MRI apparatus mayinclude a plurality of main magnetic field coils coaxially arrangedalong an axis, and a plurality of shielding coils arranged coaxiallyalong the axis. A current within at least one of the shielding coils maybe in the same direction with a current within the main magnetic fieldcoils.

In some embodiments, the shielding coils may include a first coil and asecond coil with different sizes.

In some embodiments, a direction of a current within the first coil maybe opposite to a direction of a current within the second coil.

In some embodiments, a radius of the first coil may be greater than aradius of the second coil which is concentric with the first coil.

In some embodiments, the direction of the current within the first coilmay be the same as a direction of a current within the main magneticfield coils.

According to another aspect of the present disclosure, a therapeuticapparatus is provided. The therapeutic apparatus may include a magneticresonance imaging apparatus configured to acquire magnetic resonanceimaging data with respect to a region of interest. The magneticresonance imaging apparatus may include an annular cryostat in which aplurality of main magnetic field coils and a plurality of shieldingcoils are arranged coaxially along an axis of the annular cryostat. Thetherapeutic apparatus may also include a radiation therapy apparatus.The radiation therapy apparatus may include a radiation source fordirecting therapeutic radiation to at least one portion of the region ofinterest. The annular cryostat may include a recess at an outer wall,and at least a portion of the radiation source is within the recess. Atleast one of the shielding coils may be configured to reduce themagnetic field on a region within the recess.

In some embodiments, at least one shieling coil of the shielding coilsmay be arranged close to a bottom of the recess, and a current withinthe at least one shieling coil of the shielding coils may be in anopposite direction with a current within the main magnetic field coils.

In some embodiments, at least one shieling coil of the shielding coilsmay be arranged close to an opening of the recess, and a current withinthe at least one shieling coil of the shielding coils may be in the samedirection with a current within the main magnetic field coils.

In some embodiments, the shielding coils may include a first coil and asecond coil with different sizes.

In some embodiments, a direction of a current within the first coil maybe opposite to a direction of a current within the second coil.

In some embodiments, a radius of the first coil may be greater than aradius of the second coil which is concentric with the first coil.

In some embodiments, the direction of a current within the first coilmay be the same as a direction of a current within the main magneticfield coils.

In some embodiments, at least portion of the annular recess may belocated radially between the first coil and the second coil.

According to another aspect of the present disclosure, a magneticresonance imaging (MRI) apparatus configured to acquire MRI data withrespect to a region of interest (ROI) is provided. The MRI apparatus mayinclude an annular cryostat, a plurality of main magnetic field coilscoaxially arranged along an axis of the annular cryostat, at least afirst pair of shielding coils and a second pair of shielding coils withdifferent sizes. A direction of a current within the first pair ofshielding coils may be opposite to a direction of a current within themain magnetic field coils. The first pair of shielding coils may beconfigured to shield a magnetic field outside the MRI apparatus, and thesecond pair of shielding coils may be configured to shield a magneticfield between the first pair of shielding coils and the main magneticfield coils.

In some embodiments, a direction of a current within the second pair ofshielding coils may be the same as the direction of a current within themain magnetic field coils. The second pair of shielding coils may beclose to the first pair of shielding coils.

In some embodiments, a direction of a current within the second pair ofshielding coils may be opposite to the direction of a current within themain magnetic field coils. The second pair of shielding coils may beclose to the main magnetic field coils.

In some embodiments, the apparatus may further include a third pair ofshielding coils. A shielding coil of the third pair of shielding coilsmay be concentric with a shielding coil of the second pair of shieldingcoils.

In some embodiments, a direction of a current within the third pair ofshielding coils may be opposite to a direction of a current within thesecond pair of shielding coils.

Additional features will be set forth in part in the description whichfollows, and in part will become apparent to those skilled in the artupon examination of the following and the accompanying drawings or maybe learned by production or operation of the examples. The features ofthe present disclosure may be realized and attained by practice or useof various aspects of the methodologies, instrumentalities, andcombinations set forth in the detailed examples discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is further described in terms of exemplaryembodiments. These exemplary embodiments are described in detail withreference to the drawings. These embodiments are non-limiting exemplaryembodiments, in which like reference numerals represent similarstructures throughout the several views of the drawings, and wherein:

FIG. 1 is a block diagram illustrating an exemplary radiation therapysystem according to some embodiments of the present disclosure;

FIG. 2 is a flowchart illustrating an exemplary process for applying atherapeutic radiation in a radiation therapy system according to someembodiments of the present disclosure;

FIG. 3A illustrates an exemplary therapeutic apparatus according to someembodiments of the present disclosure;

FIG. 3B illustrates another exemplary therapeutic apparatus according tosome embodiments of the present disclosure;

FIG. 4A shows an upper portion of a cross-sectional view of an exemplarytherapeutic apparatus viewed along the Z direction according to someembodiments of the present disclosure;

FIG. 4B shows an upper portion of a cross-sectional view of anotherexemplary therapeutic apparatus viewed along the Z direction accordingto some embodiments of the present disclosure;

FIG. 4C shows an upper portion of a cross-sectional view of anotherexemplary therapeutic apparatus viewed along the Z direction accordingto some embodiments of the present disclosure;

FIG. 5A shows a schematic diagram illustrating exemplary magnetic fieldsproduced by a plurality of main magnetic field coils without a shieldingcoil group according to some embodiments of the present disclosure;

FIG. 5B shows a schematic diagram illustrating exemplary magnetic fieldsproduced by a plurality of main magnetic field coils with a shieldingcoil group according to some embodiments of the present disclosure;

FIG. 6A shows a schematic diagram illustrating an exemplary curve of themagnetic field in the annular region without a shielding coil groupaccording to some embodiments of the present disclosure;

FIG. 6B shows a schematic diagram illustrating an exemplary curve of themagnetic field in the annular region with a shielding coil groupaccording to some embodiments of the present disclosure; and

FIG. 7 shows a schematic diagram illustrating an exemplary curve of themain magnetic field B0 in the MRI apparatus with a shielding coil groupaccording to some embodiments of the present disclosure.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the present disclosure, and is provided in thecontext of a particular application and its requirements. Variousmodifications to the disclosed embodiments will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to other embodiments and applications without departing fromthe spirit and scope of the present disclosure. Thus, the presentdisclosure is not limited to the embodiments shown, but is to beaccorded the widest scope consistent with the claims.

The terminology used herein is for the purpose of describing particularexemplary embodiments only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprise,”“comprises,” and/or “comprising,” “include,” “includes,” and/or“including,” when used in this specification, specify the presence ofstated features, integers, steps, operations, elements, and/orcomponents, but do not preclude the presence or addition of one or moreother features, integers, steps, operations, elements, components,and/or groups thereof.

These and other features, and characteristics of the present disclosure,as well as the methods of operation and functions of the relatedelements of structure and the combination of parts and economies ofmanufacture, may become more apparent upon consideration of thefollowing description with reference to the accompanying drawings, allof which form a part of the present disclosure. It is to be expresslyunderstood, however, that the drawings are for the purpose ofillustration and description only and are not intended to limit thescope of the present disclosure. It is understood that the drawings arenot to scale.

FIG. 1 is a block diagram illustrating an exemplary radiation therapysystem 100 according to some embodiments of the present disclosure. Insome embodiments, the radiation therapy system 100 may be amulti-modality imaging system including, for example, a positronemission tomography-radiotherapy (PET-RT) system, a magnetic resonanceimaging-radiotherapy (MRI-RT) system, etc. For better understanding thepresent disclosure, an MRI-RT system may be described as an example ofthe radiation therapy system 100, and not intended to limit the scope ofthe present disclosure.

As shown in FIG. 1 , the radiation therapy system 100 may include atherapeutic apparatus 110, one or more processing engines 120, a network130, a storage device 140, and one or more terminal devices 150. In someembodiments, the therapeutic apparatus 110, the one or more processingengines 120, the storage device 140, and/or the terminal device 150 maybe connected to and/or communicate with each other via a wirelessconnection (e.g., the wireless connection provided by the network 130),a wired connection (e.g., the wired connection provided by the network130), or any combination thereof.

The therapeutic apparatus 110 may include a magnetic resonance imagingcomponent (hereinafter referred to as “MRI apparatus”). The MRIapparatus may generate image data associated with magnetic resonancesignals (hereinafter referred to as “MRI signals”) via scanning asubject or a part of the subject. In some embodiments, the subject mayinclude a body, a substance, an object, or the like, or any combinationthereof. In some embodiments, the subject may include a specific portionof a body, a specific organ, or a specific tissue, such as head, brain,neck, body, shoulder, arm, thorax, cardiac, stomach, blood vessel, softtissue, knee, feet, or the like, or any combination thereof. In someembodiments, the therapeutic apparatus 110 may transmit the image datavia the network 130 to the one or more processing engines 120, thestorage device 140, and/or the terminal device 150 for furtherprocessing. For example, the image data may be sent to the one or moreprocessing engines 120 for generating an MRI image, or may be stored inthe storage device 140.

The therapeutic apparatus 110 may also include a radiation therapycomponent (hereinafter referred to as “radiation therapy apparatus”).The radiation therapy apparatus may provide radiation for target region(e.g., a tumor) treatment. The radiation used herein may include aparticle ray, a photon ray, etc. The particle ray may include neutron,proton, electron, p-meson, heavy ion, a-ray, or the like, or anycombination thereof. The photon ray may include X-ray, y-ray,ultraviolet, laser, or the like, or any combination thereof. Forillustration purposes, a radiation therapy apparatus associated withX-ray may be described as an example. In some embodiments, thetherapeutic apparatus 110 may generate a certain dose of X-rays toperform radiotherapy under the assistance of the image data provided bythe MRI apparatus. For example, the image data may be processed tolocate a tumor and/or determine the dose of X-rays.

The one or more processing engines 120 may process data and/orinformation obtained from the therapeutic apparatus 110, the storagedevice 140, and/or the terminal device 150. For example, the one or moreprocessing engines 120 may process image data and reconstruct at leastone MRI image based on the image data. As another example, the one ormore processing engines 120 may determine the position of the treatmentregion and the dose of radiation based on the at least one MRI image.The MRI image may provide advantages including, for example, superiorsoft-tissue contrast, high resolution, geometric accuracy, which mayallow accurate positioning of the treatment region. The MRI image may beused to detect the variance of the treatment region (e.g., a tumorregression or metastasis) during the time when the treatment plan isdetermined and the time when the treatment is carried out, such that anoriginal treatment plan may be adjusted accordingly. The originaltreatment plan may be determined before the treatment begins. Forinstance, the original treatment plan may be determined at least oneday, or three days, or a week, or two weeks, or a month, etc., beforethe treatment commences.

In the original or adjusted treatment plan, the dose of radiation may bedetermined according to, for example, synthetic electron densityinformation. In some embodiments, the synthetic electron densityinformation may be generated based on the MRI image.

In some embodiments, the one or more processing engines 120 may be asingle processing engine that communicates with and process data fromthe MRI apparatus and the radiation therapy apparatus of the therapeuticapparatus 110. Alternatively, the one or more processing engines 120 mayinclude at least two processing engines. One of the at least twoprocessing engines may communicate with and process data from the MRIapparatus of the therapeutic apparatus 110, and another one of the atleast two processing engines may communicate with and process data fromthe radiation therapy apparatus of the therapeutic apparatus 110. Insome embodiments, the one or more processing engines 120 may include atreatment planning system. The at least two processing engines maycommunicate with each other.

In some embodiments, the one or more processing engines 120 may be asingle server or a server group. The server group may be centralized ordistributed. In some embodiments, the one or more processing engines 120may be local to or remote from the therapeutic apparatus 110. Forexample, the one or more processing engines 120 may access informationand/or data from the therapeutic apparatus 110, the storage device 140,and/or the terminal device 150 via the network 130. As another example,the one or more processing engines 120 may be directly connected to thetherapeutic apparatus 110, the terminal device 150, and/or the storagedevice 140 to access information and/or data. In some embodiments, theone or more processing engines 120 may be implemented on a cloudplatform. The cloud platform may include a private cloud, a publiccloud, a hybrid cloud, a community cloud, a distributed cloud, aninter-cloud, a multi-cloud, or the like, or any combination thereof.

The network 130 may include any suitable network that can facilitate theexchange of information and/or data for the radiation therapy system100. In some embodiments, one or more components of the radiationtherapy system 100 (e.g., the therapeutic apparatus 110, the one or moreprocessing engines 120, the storage device 140, or the terminal device150) may communicate information and/or data with one or more othercomponents of the radiation therapy system 100 via the network 130. Forexample, the one or more processing engines 120 may obtain image datafrom the therapeutic apparatus 110 via the network 130. As anotherexample, the one or more processing engines 120 may obtain userinstructions from the terminal device 150 via the network 130. Thenetwork 130 may include a public network (e.g., the Internet), a privatenetwork (e.g., a local area network (LAN), a wide area network (WAN)), awired network (e.g., an Ethernet network), a wireless network (e.g., an802.11 network, a Wi-Fi network), a cellular network (e.g., a Long TermEvolution (LTE) network), a frame relay network, a virtual privatenetwork (“VPN”), a satellite network, a telephone network, routers,hubs, switches, server computers, or the like, or any combinationthereof. In some embodiments, the network 130 may include one or morenetwork access points. For example, the network 130 may include wiredand/or wireless network access points such as base stations and/orinternet exchange points through which one or more components of theradiation therapy system 100 may be connected to the network 130 toexchange data and/or information.

The storage device 140 may store data, instructions, and/or any otherinformation. In some embodiments, the storage device 140 may store dataobtained from the one or more processing engines 120 and/or the terminaldevice 150. In some embodiments, the storage device 140 may store dataand/or instructions that the one or more processing engines 120 mayexecute or use to perform exemplary methods described in the presentdisclosure. In some embodiments, the storage device 140 may include amass storage device, a removable storage device, a cloud based storagedevice, a volatile read-and-write memory, a read-only memory (ROM), orthe like, or any combination thereof. Exemplary mass storage may includea magnetic disk, an optical disk, a solid-state drive, etc. Exemplaryremovable storage may include a flash drive, a floppy disk, an opticaldisk, a memory card, a zip disk, a magnetic tape, etc. Exemplaryvolatile read-and-write memory may include a random access memory (RAM).Exemplary RAM may include a dynamic RAM (DRAM), a double date ratesynchronous dynamic RAM (DDR SDRAM), a static RAM (SRAM), a thyristorRAM (T-RAM), a zero-capacitor RAM (Z-RAM), etc. Exemplary ROM mayinclude a mask ROM (MROM), a programmable ROM (PROM), an erasableprogrammable ROM (EPROM), an electrically erasable programmable ROM(EEPROM), a compact disk ROM (CD-ROM), a digital versatile disk ROM,etc. In some embodiments, the storage device 140 may be implemented on acloud platform as described elsewhere in the present disclosure.

In some embodiments, the storage device 140 may be connected to thenetwork 130 to communicate with one or more other components of theradiation therapy system 100 (e.g., the one or more processing engines120 or the terminal device 150). One or more components of the radiationtherapy system 100 may access the data or instructions stored in thestorage device 140 via the network 130. In some embodiments, the storagedevice 140 may be part of the one or more processing engines 120.

The terminal device 150 may be connected to and/or communicate with thetherapeutic apparatus 110, the one or more processing engines 120,and/or the storage device 140. For example, the one or more processingengines 120 may acquire a scanning protocol from the terminal device150. As another example, the terminal device 150 may obtain image datafrom the therapeutic apparatus 110 and/or the storage device 140. Insome embodiments, the terminal device 150 may include a mobile device151, a tablet computer 152, a laptop computer 153, or the like, or anycombination thereof. For example, the mobile device 151 may include amobile phone, a personal digital assistance (PDA), a gaming device, anavigation device, a point of sale (POS) device, a laptop, a tabletcomputer, a desktop, or the like, or any combination thereof. In someembodiments, the terminal device 150 may include an input device, anoutput device, etc. The input device may include alphanumeric and otherkeys that may be input via a keyboard, a touch screen (for example, withhaptics or tactile feedback), a speech input, an eye tracking input, abrain monitoring system, or any other comparable input mechanism. Theinput information received through the input device may be transmittedto the one or more processing engines 120 via, for example, a bus, forfurther processing. Other types of the input device may include a cursorcontrol device, such as a mouse, a trackball, or cursor direction keys,etc. The output device may include a display, a speaker, a printer, orthe like, or any combination thereof. In some embodiments, the terminaldevice 150 may be part of the one or more processing engines 120.

This description is intended to be illustrative, and not to limit thescope of the present disclosure. Many alternatives, modifications, andvariations will be apparent to those skilled in the art. The features,structures, methods, and characteristics of the exemplary embodimentsdescribed herein may be combined in various ways to obtain additionaland/or alternative exemplary embodiments. For example, the storagedevice 140 may be a data storage including cloud computing platforms,such as public cloud, private cloud, community, hybrid clouds, etc. Insome embodiments, the one or more processing engines 120 may beintegrated into the therapeutic apparatus 110. However, those variationsand modifications do not depart the scope of the present disclosure.

FIG. 2 is a flowchart of an exemplary process 200 for applying atherapeutic radiation by a radiation therapy system according to someembodiments of the present disclosure. In some embodiments, one or moreoperations of the process 200 illustrated in FIG. 2 may be implementedin the radiation therapy system 100 illustrated in FIG. 1 . For example,the process 200 illustrated in FIG. 2 may be stored in the storagedevice 140 in the form of instructions, and invoked and/or executed bythe one or more processing engines 120 illustrated in FIG. 1 . Forillustration purposes, the implement of the process 200 in the one ormore processing engines 120 is described herein as an example. It shallbe noted that the process 200 can also be similarly implemented in theterminal device 150.

In 202, the one or more processing engines 120 may acquire magneticresonance imaging (MRI) data with respect to a region of interest (ROI)by an MRI apparatus. The MRI data may be MR signals received by an RFcoil from a subject. More detailed description related to the MR signalsmay be found elsewhere in the present disclosure at, for example, FIG. 3and the description thereof.

In some embodiments, an ROI may refer to a treatment region associatedwith a tumor. The treatment region may be a region of a subject (e.g., abody, a substance, an object). In some embodiments, the ROI may be aspecific portion of a body, a specific organ, or a specific tissue, suchas head, brain, neck, body, shoulder, arm, thorax, cardiac, stomach,blood vessel, soft tissue, knee, feet, or the like, or any combinationthereof.

In 204, the one or more processing engines 120 may reconstruct an MRIimage related to at least one portion of the ROI based on the MRI data.The MRI image may be reconstructed as a distribution of atomic nucleiinside the subject based on the MRI data. Different kinds of imagingreconstruction techniques for the image reconstruction procedure may beemployed. Exemplary image reconstruction techniques may include Fourierreconstruction, constrained image reconstruction, regularized imagereconstruction in parallel MRI, or the like, or a variation thereof, orany combination thereof.

The MRI image may be used to determine a therapeutic radiation to atumor. For example, the one or more processing engines 120 may determinethe position of the tumor and the dose of radiation according to the MRIimage. In some embodiments, it may take at least several minutes toreconstruct an MRI image representing a large imaging region. In someembodiments, in order to generate the MRI image during a relative shorttime period (e.g., every second), the one or more processing engines 120may reconstruct an initial image representing a smaller imaging region(e.g., at least one portion of the ROI) compared to that of the MRIimage representing a large imaging region, and then combine the initialimage with the MRI image representing a large imaging region. Forexample, the one or more processing engines 120 may replace a portion ofthe MRI image representing a large imaging region related to the ROIwith the initial image. The MRI image representing a large imagingregion may include information of non-ROI (e.g., a healthy tissue) nearthe ROI and that of the ROI. In some embodiments, the MRI imagerepresenting a large imaging region may be acquired and reconstructedbefore the therapeutic radiation on the tumor. For example, the MRIimage representing a large imaging region may be acquired less than 1day, or half a day, or 6 hours, or 3 hours, or 1 hour, or 45 minutes, or30 minutes, or 20 minutes, or 15 minutes, or 10 minutes, or 5 minutes,etc., before a radiation source starts emitting a radiation beam fortreatment. In some embodiments, the radiation source may include somecomponents to generate a radiation beam. For example, the radiationsource may include a linear accelerator, a target, a primary collimatorand a multi-leaf collimator (MLC), etc. In some embodiments, the MRIimage representing a large imaging region may be obtained from a storagedevice in the radiation therapy system 100, such as the storage device140.

In 206, the one or more processing engines 120 may determine a parameterassociated with a size of the at least one portion of the ROI based onthe MRI image. In some embodiments, the parameter associated with a sizeof the at least one portion of the ROI may include the size of the crosssection of a tumor which has the maximum area and is perpendicular tothe direction of the radiation beams impinging on the at least oneportion of the ROI. In some embodiments, the parameter associated with asize of the at least one portion of the ROI may indicate the shape ofthe cross section of the tumor. For example, the parameter associatedwith a size of at least one portion of the ROI may indicate that theshape of the cross section of the tumor is circle, and further indicatethe diameter of the circle. In some embodiments, to determine theparameter associated with a size of at least one portion of the ROI, theone or more processing engines 120 may extract texture information fromthe MRI image, and determine texture features that are indicative of theROI by identifying frequent texture patterns of the ROI in the extractedtexture information. Then, the one or more processing engines 120 maymeasure the size of the region which includes the texture features inthe MRI image, and determine the parameter associated with the size ofthe ROI.

In 208, the one or more processing engines 120 may generate a controlsignal according to the parameter associated with the size of at leastone portion of the ROI. The control signal may be dynamically adjustedbased on the plurality of MRI images taken at different time points. Insome embodiments, the control signal may include parameters associatedwith the therapeutic radiation on the tumor. For example, the controlsignal may include the dosage of X-rays and a duration of the radiationbeam. For another example, the control signal may include parameters ofmulti-leaf collimator (MLC) that determines the shape of the radiationbeam projected on the subject. The MLC may include a plurality ofindividual leaves of high atomic numbered materials (e.g., tungsten)moving independently in and out of the path of the radiation beam. Insome embodiments, the control signal may include parameters associatedwith movements of one or more components of a radiation therapyapparatus. For example, the control signal may include a parameterassociated with one or more positions of a radiation source of theradiation therapy apparatus (e.g., the radiation therapy apparatus inthe therapeutic apparatus 110, a radiation therapy apparatus 300). Foranother example, the control signal may include a parameter associatedwith a height or a position of a platform of the radiation therapyapparatus (e.g., a location of the platform 308 of the treatment table330 along an axis of the main magnetic body 302) to properly position apatient so that the treatment region (e.g., a cancerous tumor or lesion)in the patient may properly receive the radiation beam from theradiation therapy apparatus.

In 210, the one or more processing engines 120 may send the controlsignal to a radiation therapy apparatus to cause the radiation therapyapparatus to apply the therapeutic radiation. During the therapeuticradiation, the radiation source of the radiation therapy apparatus mayrotate, and the dosage of X-rays, duration of radiation beam from aradiation source, the shape of MLC and the position of the platform maybe varied. In some embodiments, the radiation beam may be emitted onlywhen the radiation source of the radiation therapy apparatus rotates tocertain angles (e.g., 60 degrees, 120 degrees, 180 degrees, 240 degrees,300 degrees, 360 degrees). For example, an intensity modulated radiationtherapy (IMRT) may be applied. The radiation source may stop rotatingintermittently. The radiation source may rotate to a desired position,pause there, and emit a radiation beam, and then resume to rotate. Insome embodiments, the radiation source may rotate continuously, and emita radiation beam continuously or intermittently. In some embodiments,the radiation source may continuously emit the radiation beam whilerotating.

In some embodiments, as described above, a treatment region (e.g., aregion including a tumor) may be determined according to the image dataacquired from the MRI apparatus. Then a radiation beam may be generatedby a radiation source of the radiation therapy apparatus to perform thetherapeutic radiation to the treatment region. For example, the dosageof the radiation beam and/or the position of the treatment region may bedetermined in real-time with the assistance of the MRI apparatus.

It should be noted that the above description is merely provided for thepurposes of illustration, and not intended to limit the scope of thepresent disclosure. For persons having ordinary skills in the art,multiple variations or modifications may be made under the teachings ofthe present disclosure. However, those variations and modifications donot depart from the scope of the present disclosure. For example,operations 202 and 204 may be performed simultaneously.

FIG. 3A illustrates an exemplary therapeutic apparatus 110 according tosome embodiments of the present disclosure. As illustrated in FIG. 3A,the therapeutic apparatus 110 may include an MRI apparatus 310, aradiation therapy apparatus 300, and a treatment table 330. In someembodiments, the MRI apparatus 310 may generate the MRI data asdescribed in connection with operation 202, and the radiation therapydevice 300 may apply the therapeutic radiation as described inconnection with operation 210.

The MRI apparatus 310 may include a bore 301, a main magnetic body 302,one or more gradient coils (not shown), and one or more radiofrequency(RF) coils (not shown). The MRI apparatus 310 may be configured toacquire image data from an imaging region. For example, the image datamay relate to the treatment region associated with a tumor. In someembodiments, the MRI apparatus 310 may be a permanent magnet MRIscanner, a superconducting electromagnet MRI scanner, or a resistiveelectromagnet MRI scanner, etc., according to the types of the mainmagnetic body 302. In some embodiments, the MRI apparatus 310 may be ahigh-field MRI scanner, a mid-field MRI scanner, and a low-field MRIscanner, etc., according to the intensity of the magnetic field. In someembodiments, the MRI apparatus 310 may be of a closed-bore (cylindrical)type, an open-bore type, or the like.

The main magnetic body 302 may have the shape of an annulus and maygenerate a static magnetic field B0. The main magnetic body 302 may beof various types including, for example, a permanent magnet, asuperconducting electromagnet, a resistive electromagnet, etc. Thesuperconducting electromagnet may include niobium, vanadium, technetiumalloy, etc.

The one or more gradient coils may generate magnetic field gradients tothe main magnetic field B0 in the X, Y, and/or Z directions (or axes).In some embodiments, the one or more gradient coils may include anX-direction (or axis) coil, a Y-direction (or axis) coil, a Z-direction(or axis) coil, etc. For example, the Y-direction coil may be designedbased on a circular (Maxwell) coil, the Z-direction coil and theX-direction coil may be designed on the basis of the saddle (Golay) coilconfiguration. As used herein, the Z direction may also be referred toas the readout (RO) direction (or a frequency encoding direction), the Xdirection may also be referred to as the phase encoding (PE) direction,the Y direction may also be referred to as the slice-selection encodingdirection. In the present disclosure, the readout direction and thefrequency encoding direction may be used interchangeably.

Merely by way of example, the gradient magnetic fields may include aslice-selection gradient field corresponding to the Y-direction, a phaseencoding (PE) gradient field corresponding to the X-direction, a readout(RO) gradient field corresponding to the Z-direction, etc. The gradientmagnetic fields in different directions may be used to encode thespatial information of MR signals. In some embodiments, the gradientmagnetic fields may also be used to perform at least one function offlow encoding, flow compensation, flow dephasing, or the like, or anycombination thereof.

The one or more RF coils may emit RF pulses to and/or receive MR signalsfrom a subject (e.g., a body, a substance, an object) being examined. Asused herein, an RF pulse may include an excitation RF pulse and arefocusing RF pulse. In some embodiments, the excitation RF pulse (e.g.,a 90-degree RF pulse) may tip magnetization vector away from thedirection of the main magnetic field B0. In some embodiments, therefocusing pulse (e.g., a 180-degree RF pulse) may rotate dispersingspin isochromatic about an axis in the transverse plane so thatmagnetization vector may rephase at a later time. In some embodiments,the RF coil may include an RF transmitting coil and an RF receivingcoil. The RF transmitting coil may emit RF pulse signals that may excitethe nucleus in the subject to resonate at the Larmor frequency. The RFreceiving coil may receive MR signals emitted from the subject. In someembodiments, the RF transmitting coil and RF receiving coil may beintegrated into one single coil, for example, a transmitting/receivingcoil. The RF coil may be one of various types including, for example, aquotient difference (QD) orthogonal coil, a phase-array coil, etc. Insome embodiments, different RF coils 240 may be used for the scanning ofdifferent parts of a body being examined, for example, a head coil, aknee joint coil, a cervical vertebra coil, a thoracic vertebra coil, atemporomandibular joint (TMJ) coil, etc. In some embodiments, accordingto its function and/or size, the RF coil may be classified as a volumecoil and a local coil. For example, the volume coil may include abirdcage coil, a transverse electromagnetic coil, a surface coil, etc.As another example, the local coil may include a solenoid coil, a saddlecoil, a flexible coil, etc.

The radiation therapy device 300 may include a drum 312 and a base 307.The drum 312 may have the shape of an annulus. The drum 312 may bedisposed around the main magnetic body 302 and intersect the mainmagnetic body 302 at a central region of the main magnetic body 302along the axis 311 of the bore 301. The drum 312 may accommodate andsupport a radiation source that is configured to emit a radiation beamtowards the treatment region in the bore 301. The radiation beam may bean X-ray beam, an electron beam, a proton ray source, etc. The drum 312,together with the radiation source mounted thereon, may be able torotate around the axis 311 of the bore 301 and/or a point called theisocenter. Merely by way of example, the drum 312, together with theradiation source mounted thereon, may be able to rotate any angle, e.g.,90 degrees, 180 degrees, 360 degrees, 450 degrees, 540 degrees, aroundthe axis 311. The drum 306 may be further supported by the base 307.

It should be noted that the above description is merely provided for thepurposes of illustration, and not intended to limit the scope of thepresent disclosure. For persons having ordinary skills in the art,multiple variations or modification may be made under the teaching ofthe present disclosure. For example, the radiation therapy device 300may further include a linear accelerator configured to accelerateelectrons, ions, or protons, a dose detecting device, a temperaturecontrolling device (e.g., a cooling device), a multiple layercollimator, or the like, or any combination thereof. However, thosevariations and modifications do not depart from the scope of the presentdisclosure.

The treatment table 330 may include a platform 308 and a base frame 309.In some embodiments, the platform 308 may move along the horizontaldirection and enter into the bore 301 of the MRI apparatus 310. In someembodiments, the platform 308 may move two-dimensionally,three-dimensionally, four-dimensionally, five-dimensionally orsix-dimensionally. In some embodiments, the platform 308 may moveaccording to the variance (e.g., position change) of the tumor estimatedby, for example, a real-time MRI image obtained during a treatment.

In some embodiments, the subject may be placed on the platform 308 andsent into the MRI device. In some embodiments, the subject may be ahuman patient. The human patient may lie on the back, lie in prone, lieon the side on the platform 308.

During the treatment, the drum 312 may be set to rotate around the mainmagnetic body 302. In some embodiments, the main magnetic body 302 mayinclude a recess (not shown) at its outer wall. The recess may bedisposed around the entire circumference of the main magnetic body 302.For example, the recess may have the shape of an annulus surrounding themain magnetic body 302, thus accommodating at least part of the drum312. In some embodiments, the recess may be disposed around part of thecircumference of the main magnetic body 302. For example, the recess mayhave the shape of one or more arcs around the main magnetic body 302.

In some embodiments, at least a portion of the radiation source iswithin the recess. This arrangement may reduce the distance between theradiation source and the axis 311 of the bore 301 along the radialdirection of the main magnetic body 302. In some embodiments, theradiation source may move along an entire path of rotation within therecess. In some embodiments, the radiation source may move along a pathof rotation within the recess that is not an entire circle, such as asemicircle or ¾ circle or ⅘ circle. Under such situation, the radiationsource will move clockwise and then anti-clockwise during treatment, andthe table may also move. The radiation source may generate the radiationbeam according to one or more parameters. Exemplary parameter mayinclude a parameter of the radiation beam, a parameter of the radiationsource, or a parameter of the platform 308. For example, the parameterof the radiation beam may include an irradiating intensity, anirradiating angle, an irradiating distance, an irradiating area, anirradiating time, an intensity distribution, or the like, or anycombination thereof. The parameter of the radiation source may include aposition, a rotating angle, a rotating speed, a rotating direction, theconfiguration of the radiation source, or the like, or any combinationthereof. In some embodiments, the generation of the radiation beam bythe radiation source may take into consideration energy loss of theradiation beam due to, e.g., the main magnetic body 302 located in thepathway of the radiation beam that may absorb at least a portion of theradiation beam. For example, the irradiating intensity of the radiationbeam may be set larger than that in the situation in which there is noenergy loss due to, e.g., the absorption by the main magnetic body 302accordingly to compensate the energy loss such that the radiation beamof a specific intensity may impinge on a treatment region (e.g., atumor).

FIG. 3B illustrates another exemplary therapeutic apparatus 110′according to some embodiments of the present disclosure. Compared withthe therapeutic apparatus 110 described in FIG. 3A, the therapeuticapparatus 110′ may use a gantry 306 instead of the drum 312. The gantry306 may be disposed at one side of the main magnetic body 302. Atreatment head 304 may be installed on the gantry 306 via a treatmentarm 305. The treatment head 304 may accommodate the radiation source.The gantry 306 may be able to rotate the treatment head 304 around theaxis 311 of the bore 301.

As shown in FIG. 3B, a recess 303 may be formed at the outer wall of themain magnetic body 302 and have the shape of an annulus. The recess 303may accommodate at least a portion of the treatment head 304 and providea path for rotation of the treatment head 304. This arrangement mayreduce the distance between the treatment head 304 and the axis 311 ofthe bore 301 along the radial direction of the main magnetic body 302.In some embodiments, the reduction of the distance between the treatmenthead 304 and the axis 311 of the bore 301 may cause an increase of theradiation dose that may reach the treatment region (e.g., a tumor) whichleads to an enhancement in the therapeutic efficiency. In someembodiments, the width of the recess 303 along the Y direction (i.e.,the axial direction of the main magnetic body 302) may be no less thanthe width of the treatment head 304 along the Y direction.

It should be noted that the above description of the therapeuticapparatus 110 is merely provided for the purposes of illustration, andnot intended to limit the scope of the present disclosure. For personshaving ordinary skills in the art, multiple variations and modificationsmay be made under the teachings of the present disclosure. For example,the assembly and/or function of the therapeutic apparatus 110 may varyor change according to a specific implementation scenario. In someembodiments, the main magnetic body 302 of the MRI apparatus 310 mayalso rotate relative to the treatment head 304. For example, theradiation therapy device 300 and the MRI apparatus 310 may synchronouslyor asynchronously rotate around a same axis (e.g., the axis 311).However, those variations and modifications do not depart from the scopeof the present disclosure.

FIG. 4A shows an upper portion of a cross-sectional view of an exemplarytherapeutic apparatus 400 viewed along the Z direction according to someembodiments of the present disclosure. The therapeutic apparatus 400 mayinclude an MRI apparatus that is configured to generate MRI data and aradiation therapy apparatus that is configured to apply therapeuticradiation.

As shown in FIG. 4A, the MRI apparatus may include a plurality of mainmagnetic field coils 401 (e.g., first main magnetic field coils 401-1,second main magnetic field coils 401-2, third main magnetic field coils401-3), a plurality of shielding coils (e.g., shielding coils 402,shielding coils 411-1, shielding coils 411-2), and a cryostat 403. Theshielding coils 402 may include a first pair of shielding coils with afirst size, i.e., a first shielding coil 402-a and a second shieldingcoil 402-b. The shielding coils 411-1 may include a second pair ofshielding coils with a second size. The shielding coils 411-2 mayinclude a third pair of shielding coils with a third size. The firstsize, the second size and the third size may be different from eachother. The shielding coils 411-1 (i.e., the second pair of shieldingcoils) may be close to the shielding coils 402 (i.e., the first pair ofshielding coils). In some embodiments, the shielding coils 411-1 (alsoreferred to as first coils) and the shielding coils 411-2 (also referredto as second coils) may also be referred as a shielding coil group 411.

The plurality of main magnetic field coils 401, the shielding coils 402and the shielding coil group 411 may be accommodated in the cryostat 403and maintained in the superconductive state under a certain condition(e.g., when both the coils are merged in a cooling medium in thecryostat 403).

The cryostat 403 may have the shape of an annulus with an axis 405(e.g., the axis 311 in FIG. 3A). The plurality of main magnetic fieldcoils 401 may be arranged coaxially along the axis 405 to generate auniform magnetic field (e.g., a static magnetic field B0) within aspecific region (e.g., the region within the bore 301) when theplurality of main magnetic field coils 401 carry an electric currentalong a first direction. In some embodiments, the first main magneticfield coils 401-1, the second main magnetic field coils 401-2, and thethird main magnetic field coils 401-3 may have the same radius ordifferent radiuses.

The shielding coils 402 may also be arranged coaxially along the axis405 at a larger radius from the axis 405 than the plurality of mainmagnetic field coils 401. That is, a radius of each of the firstshielding coil 402-a and the second shielding coil 402-b may be largerthan that of each of the plurality of main magnetic field coils 401. Theshielding coils 402 may carry an electric current along a seconddirection that is opposed to the first direction. The shielding coils402 (i.e., the first pair of shielding coils) may help shield themagnetic field generated by the plurality of main magnetic field coils401 on a region outside the MRI apparatus.

The shielding coil group 411 may also be arranged coaxially along theaxis 405 at a larger radius from the axis 405 than the plurality of mainmagnetic field coils 401. That is, a radius of each of the first coils411-1 and second coils 411-2 may be larger than that of each of theplurality of main magnetic field coils 401. A direction of a currentwithin each of the first coils 411-1 may be opposite to a direction of acurrent within each of the second coils 411-2. For example, each of thefirst coils 411-1 may include a radius designated as R1, and each of thesecond coils 411-2 may include a radius designated as R2, wherein R1 isgreater than R2. Each of the first coils 411-1 may carry an electriccurrent along the first direction, and each of the second coils 411-2may carry an electric current along the second direction. That is, thedirection of electric current within the first coils 411-1 (i.e., thesecond pair of shielding coils) may be the same as that of the pluralityof main magnetic field coils 401, and the direction of electric currentwithin the second coils 411-2 (i.e., the third pair of shielding coils)may be opposite to that of the plurality of main magnetic field coils401 (i.e., the direction of a current within the third pair of shieldingcoils may be opposite to the direction of a current within the secondpair of shielding coils). In some embodiments, a shielding coil of thesecond pair of shielding coils (i.e., a first coil 411-1) may beconcentric with a shielding coil of the third pair of shielding coils(i.e., a second coil 411-2). The first coil 411-1 and the second coil411-2 that are arranged concentrically may also be referred as a coilgroup of the shielding coil group 411. As shown in FIG. 4A, theshielding coil group 411 may include a first coil group and a secondcoil group.

In some embodiments, the shielding coil group 411 may be configured toshield the magnetic field produced by the MRI apparatus (e.g., the mainmagnetic field coils, the magnetic shielding coils, the gradient coils)in case that one or more components of the radiation therapy apparatus(e.g., a linear accelerator, electrons, a multi-leaf collimator) may beinfluenced by the magnetic field produced by the MRI apparatus on anannular region. The annular region may have the shape of an annulus withthe axis 405. The annular region may include a virtual outer wall with aradius of R1 and a virtual inner wall with a radius of R2. That is, thedepth of the annular region (i.e., the thickness of the annular regionin the radial direction) which is defined as the distance from thevirtual outer wall to the virtual inner wall in the radial direction maybe equal to R1 minus R2 (R1−R2). For example, the shielding coil group411 (e.g., the second pair of shielding coils 411-1, or the third pairof shielding coils 411-2) may be configured to shield a magnetic fieldbetween the shielding coils 402 (i.e., the first pair of shieldingcoils) and the main magnetic field coils 401. As another example, theshielding coil group 411 (e.g., the second pair of shielding coils411-1, the third pair of shielding coils 411-2) may be configured toreduce a magnetic field on a region within a recess (e.g., a recess 408)of the annular cryostat 403.

In some embodiments, a magnitude of the electric current in each coil ofthe shielding coil group 411 may be the same, i.e., each of the firstcoils 411-1 may have the same magnitude of electric current as each ofthe second coils 411-2. Taking the first direction that is perpendicularto the X-Y plane pointing inwards as an example, the second directionmay be perpendicular to the X-Y plane pointing outward. For the annularregion, the magnetic field produced by the plurality of main magneticfield coils 401 (also referred to as a first magnetic field) in theannular region may be along the Y direction, and the magnetic fieldproduced by the shielding coil group 411 (also referred to as the secondmagnetic field) in the annular region may be opposite to the Ydirection. The magnitude of the first magnetic field may be equal to orapproximately equal to the second magnetic field by adjusting themagnitude of the electric current in each coil of the shielding coilgroup 411 to a proper magnitude. In the proper magnitude of the electriccurrent in each coil of the shielding coil group 411, the first magneticfield and the second magnetic field may neutralize each other such thatthe magnetic field in the annular region may be equal to or less than athreshold field (e.g., a zero net field). The threshold field may be setby an operator or a default setting of the radiation therapy system 100,and may be adjustable in different situations. For a region of the mainmagnetic field B0 that produced by the plurality of main magnetic fieldcoils 401, the magnetic field produced by the shielding coil group 411(also referred to as the third magnetic field) in the region of the mainmagnetic field B0 may be a magnetic field equal to or less than thethreshold field, as the first coils 411-1 and the second coils 411-2 mayproduce two magnetic fields with approximate magnitudes and oppositedirections in the region of the main magnetic field B0 and the twomagnetic fields may neutralize each other. Thus, the main magnetic fieldB0 may not be influence by the protection.

As shown in FIG. 4A, the cryostat 403 may include two chambers (e.g.,the left chamber and the right chamber for brevity). The two chambersmay be located at opposite sides of the cryostat 403 along the axialdirection (i.e., the direction of the axis 405) and may be connected bya neck portion between the two chambers. The neck portion may have asmaller radial size than the two chambers. Each chamber may have theshape of an annulus with a different outer wall. In some embodiments,the outer wall may refer to the outermost surface of each chamber thathas the shape of a ring. The two chambers and the neck portion may sharea same inner wall, i.e., the inner wall of the cryostat 403. In someembodiments, the inner wall may refer to the innermost surface of eachchamber that also has the shape of a ring. In some embodiments, eachchamber may accommodate at least one of the plurality of main magneticfield coils 401, at least one of the shielding coils 402, and at leastone of the first coils 411-1 and the second coils 411-2 of the shieldingcoil group 411. For example, at least one of the plurality of mainmagnetic field coils 401 may be arranged near the inner wall of the leftchamber, at least one of the shielding coils 402 (e.g., the firstshielding coil 402-a) may be arranged near the outer wall of the leftchamber, at least one of the first coils 411-1 and the second coils411-2 of the shielding coil group 411 (e.g., the first coil group) maybe arranged near the outer wall of the left chamber and closely to theneck portion. A gap 406 may be formed between the main magnetic fieldcoils arranged in the left chamber and the main magnetic field coilsarranged in the right chamber, allowing the radiation beam produced bythe radiation therapy apparatus to pass through. The two chambers may bein fluid communication with each other through the neck portion betweenthem. The cryostat 403 may contain cooling mediums in which theplurality of main magnetic field coils 401 and the shielding coils 402are merged to achieve the superconducting state.

The cryostat 403 may have a recess 408 at a radial position between theinner wall of the cryostat 403 and the outer walls of the differentchambers of the cryostat 403. The recess 408 may have an opening 407formed between the outer walls of the two chambers of the cryostat 403.The recess 408 may have the shape of an annulus when viewed in aperspective view. The annulus may have same or different widths (i.e.,the size in the axial direction) at different radial positions. Therecess 408 may have a depth (i.e., the thickness of the annulus in theradial direction) which is defined as the distance from the opening 407to the outermost surface of the neck portion of the cryostat 403 in theradial direction. As show in FIG. 4A, the third pair of shielding coilsmay be arranged close to a bottom of the recess 408, and the second pairof shielding coils may be arranged close to the opening of the recess408.

The recess 408 may be configured to accommodate the components of theradiation therapy apparatus. As shown in FIG. 4A, the recess 408 mayaccommodate at least a portion of a radiation source, wherein theradiation source includes a linear accelerator 409, a collimator 412, atarget 404 and a multi-leaf collimator (MLC) 410.

The linear accelerator 409 may be configured to accelerate chargedsubatomic particles or ions to a high speed. In some embodiments, thelinear accelerator 409 may accelerate electrons using microwavetechnology. For example, the linear accelerator 409 may accelerateelectrons in an electron beam with energy group between 4 MeV to 22 MeVusing high RF electromagnetic waves.

The linear accelerator 409 may be mounted to a gantry or a drum (e.g.,the gantry 306 or the drum 312) that is capable of rotating around theaxis 405 and may enable the radiation beam to be emitted from a certainrange of the circumferential positions, or an arbitrary circumferentialposition. As shown in FIG. 4A, the gantry or the drum may rotate to afirst position where the linear accelerator 409 may be located above theaxis 405. The linear accelerator 409 may include an acceleratingwaveguide (tube) whose axis is perpendicular to the axis 405. Theaccelerating waveguide (tube) may provide a linear path for acceleratingthe electrons along a beam path that is perpendicular to the axis 405.The one skilled in the art could readily understand that electronsdescribed herein could be replaced by other particles in otherembodiments.

The target 404 may be configured to receive the accelerated chargedsubatomic particles or ions (e.g., an electron beam) to produce theradiation beam for the therapeutic radiation. For example, the electronbeam may collide with the target 404 to generate high-energy X-raysaccording to the bremsstrahlung effect. In some embodiments, the target404 may be located near the exit window of the linear accelerator 409 toreceive the accelerated electron beam. In some embodiments, the target404 may be made of materials including aluminum, copper, silver,tungsten, or the like, or any combination thereof. Alternatively, thetarget 404 may be made of composite materials including tungsten andcopper, tungsten and silver, tungsten and aluminum, or the like, or anycombination thereof. The one skilled the art could readily understandthat the target is not necessary for the treatment using the electronbeam.

The radiation beam from the target 404 may pass through the collimator412 to form a beam with a specific shape (e.g., cone beam). In someembodiments, the collimator 412 may include a primary collimator, aflattening filter and at least one secondary collimator.

The MLC 410 may be configured to reshape the radiation beam. Forexample, the MLC 410 may adjust the irradiating shape, the irradiatingarea, etc., of the radiation beam. The MLC 410 may be placed anywhere onthe path of the radiation beam. For example, the MLC 410 may be placedclose to the linear accelerator 409 as shown in FIG. 4A. Thus, theradiation beam, after being reshaped by the MLC 410, may further passthrough the neck portion of the cryostat 403 and the gap 406 between theplurality of main magnetic field coils to arrive at the treatmentregion. As another example, the MLC 410 may be placed at a relativelylong distance away from the linear accelerator (e.g., as such that theMLC 410 may be closer to, e.g., the patient to be radiated.

The MLC 410 may stay fixed relative to the linear accelerator 409, thusrotating together with the linear accelerator 409 around the axis 405.The MLC 410 may include a plurality of individual leaves of high atomicnumbered materials (e.g., tungsten) moving independently in and out ofthe path of the radiation beam in order to block it. The shape of theradiation beam may vary when the plurality of individual leaves move inand out, forming different slots that could adapt the cross section ofthe tumor viewed from an axis of the radiation beam (i.e., the verticaldotted line 416 shown in FIG. 4A). In some embodiments, the MLC 410 mayinclude one or more layers of leaves. For example, the MLC 410 may haveonly one layer of leaves and the height of the MLC 410 along the axis ofthe radiation beam from the top of the MLC 410 to the bottom of the MLC410 may be between 7 and 10 centimeters. For another example, the MLC410 may include two layers and the height of the MLC 410 may be at least15 centimeters.

As shown in FIG. 4A, the radiation therapy apparatus may be locatedcoaxially and/or radially between the first coil group and the secondcoil group. The radiation therapy apparatus may rotate within theannular region such that all components of the radiation therapyapparatus (e.g., the linear accelerator 409, the collimator 412, thetarget 404, the MLC 410) may not be influenced by the magnetic fieldproduced by the MRI apparatus as possible. The depth of the annularregion (i.e., R1-R2) may be equal to or greater than a height of aportion of the radiation therapy apparatus (e.g., a height of at least aportion of the radiation source) which is defined as the distance fromthe top of the portion of the radiation therapy apparatus to the bottomof the portion of radiation therapy apparatus in the radial direction.

In some embodiments, the depth of the annular region may onlyaccommodate a portion of components of the radiation therapy apparatusto protect the portion of components from being influenced by themagnetic field produced by the MRI apparatus as possible. For example,the annular region may accommodate the target 404, the collimator 412and the MLC 410. The linear accelerator 409 may be out of the annularregion, as the accelerating waveguide (tube) of the linear accelerator409 may be surrounded by a shielding structure or the linear accelerator409 may be located in a relatively long distance away from the pluralityof main magnetic field coils 401. The shielding structure may include aplurality of shielding layers to shield the magnetic field produced bythe MRI apparatus in case that the electrons may be influenced by themagnetic field and/or absorb the radiation produced by the radiationbeam of the linear accelerator 409 in case that the plurality of mainmagnetic field coils 401 is influenced. As another example, the annularregion may accommodate the linear accelerator 409 and the target 404.The collimator 412 and the MLC 410 may be out of the annular region.More descriptions of therapeutic apparatus may be found in InternationalApplication No. PCT/CN2018/115394 entitled “RADIATION THERAPY SYSTEM ANDMETHOD,” filed Nov. 14, 2017, the contents of which are herebyincorporated by reference.

FIG. 4B shows an upper portion of a cross-sectional view of an exemplarytherapeutic apparatus 400′ viewed along the Z direction according tosome embodiments of the present disclosure. Compared with thetherapeutic apparatus 400 described in FIG. 4A, at least part of thelinear accelerator 409 of the therapeutic apparatus 400 may be locatedat the outside of the recess 408 along the radial direction of thecryostat 403, and a radius of each of the shielding coils 402 may rangefrom R1 to R2. As shown in FIG. 4B, the linear accelerator 409 and thetarget 404 may stretch out of the opening 407 formed by the outer wallsof the cryostat 403. The collimator 412 and the MLC 410 may beaccommodated within the annular region in case the collimator 412 andthe MLC 410 is influenced by the magnetic field produced by theplurality of main magnetic field coils. In some embodiments, the linearaccelerator 409 may be supported by or mounted to a gantry or a drum(e.g., the gantry 306 or the drum 312) that is capable of rotatingaround the axis 405. More descriptions of the therapeutic apparatus 400may be found elsewhere in the present disclosure (e.g., FIGS. 5-7 andthe descriptions thereof).

FIG. 4C shows an upper portion of a cross-sectional view of anotherexemplary therapeutic apparatus 400″ viewed along the Z directionaccording to some embodiments of the present disclosure. The therapeuticapparatus 400″ may include an MRI apparatus that is configured togenerate MRI data and a radiation therapy apparatus that is configuredto apply therapeutic radiation.

As shown in FIG. 4C, the MRI apparatus may include a main magnet bodyand a plurality of shielding coils. For illustration, the main magnetbody may include a plurality of main magnetic field coils 401 (e.g.,first main magnetic field coils 401-1, second main magnetic field coils401-2, third main magnetic field coils 401-3), and the MRI apparatus mayalso include a cryostat 403. The plurality of main magnetic field coils401 and the plurality of shielding coils may be accommodated in thecryostat 403 and maintained in the superconductive state under a certaincondition (e.g., when both the coils are merged in a cooling medium inthe cryostat 403). The plurality of shielding coils may includeshielding coils 402 and a shielding coil group 411. The shielding coils402 may include a first pair of shielding coils with a first size, i.e.,a first shielding coil 402-a and a second shielding coil 402-b. Theshielding coil group 411 may include a second pair of shielding coilswith a second size. The first size and the second size may be the sameas or different from each other.

In some embodiments, the main magnetic body and one or more of theplurality of shielding coils (e.g., the shielding coil group 411) may benot limited to a coil type, and may be any other types including, forexample, a permanent magnet, a resistive electromagnet, etc.

In some embodiments, the shielding coils 402 (i.e., the first pair ofshielding coils) may be configured to shield a magnetic field outsidethe MRI apparatus. The shielding coil group 411 (i.e., the second pairof shielding coils) may be configured to generate a magnetic field toreduce and/or shield a magnetic field in a particular region (e.g., aregion surrounding the treatment head of the radiation therapyapparatus) of the therapeutic apparatus 400″.

Compared with the MRI apparatus 400 described in FIG. 4A, the shieldinggroup 411 in FIG. 4C may include only one pair of shielding coils whilethe shielding group 411 in FIG. 4A includes two pairs of shieldingcoils. Both the shielding group 411 in FIG. 4C and the shielding group411 in FIG. 4C may be configured to shield the magnetic field producedby the MRI apparatus (e.g., the main magnetic field coils, the magneticshielding coils, the gradient coils) in case that one or more componentsof the radiation therapy apparatus (e.g., a linear accelerator,electrons, a multi-leaf collimator) may be influenced by the magneticfield produced by the MRI apparatus on the annular region thataccommodates at least a portion of a radiation source of the radiationtherapy apparatus.

In some embodiments, a direction of electric current within theshielding coil group 411 may be the same as or different from adirection of electronic current within the shielding coils 402. The mainmagnetic coils 401 may carry an electric current along a first directionand the shielding coils 402 may carry an electric current along a seconddirection. The first direction is opposite to the second direction asdescribed in FIG. 4A. Merely by way of example, if the second size isequal to the first size, the shielding coil group 411 may carry anelectronic current along the first direction, which is opposite to thatof the shielding coils 402. The electronic current within the shieldingcoil group 411 may be adjusted to a proper magnitude to achieve that amagnetic field within the annular region is equal to or less than athreshold field, such that one or more components of the radiationtherapy apparatus (e.g., a linear accelerator, electrons, a multi-leafcollimator) may not be influenced as possible. As another example, boththe second size and the electronic current within the shielding coilgroup 411 may be adjusted to proper magnitudes to achieve that themagnetic field within the annular region is equal to or less than thethreshold field. Merely by way of example, if the shielding coil group411 is arranged close to a bottom of the recess 408, a current withinthe shielding coil group 411 may be opposite to the first direction. Ifthe shielding coil group 411 is arranged close to an opening of therecess 408, a current within the shielding coil group 411 may be alongthe first direction.

In some embodiments, a distance (e.g., denoted by R as illustrated inFIG. 4C) from the axis 405 to the target 404 of the radiation therapyapparatus may be reduced to be a normal source-axis distance (e.g., lessthan or equal to 1 meter) with the shielding coil group 411, while thedistance from the axis 405 to the target 404 of the radiation therapyapparatus may be larger than the normal source-axis distance (e.g., 1meter) without the shielding coil group 411.

It should be noted that the above description of the therapeuticapparatus 400, 400′ or 400″ is merely provided for the purposes ofillustration, and not intended to limit the scope of the presentdisclosure. For persons having ordinary skills in the art, multiplevariations and modifications may be made under the teachings of thepresent disclosure. For example, the collimator 412 and the MLC 410 maybe integrated to form a single collimator. As another example, the neckportion illustrated in the cryostat 403 may not form an entire annulus.Specifically, the neck portion may be discrete arcs that connect theleft chamber and the right chamber of the cryostat 403. Therefore, theneck portion may intermittently appear in the path of the radiation beamwhen the linear accelerator 409 rotates around the axis 405 to generatethe radiation beam. As still another example, the shielding coil group411 may include more than two groups of coils as disclosed in thepresent disclosure, e.g., a third coil group may be added for achievingshielding the magnetic field in the annular region. As still anotherexample, the shielding coil group 411 may include more than two pairs ofshielding coils as disclosed in the present disclosure, e.g., a fourthpair of shielding coils may be added for achieving shielding themagnetic field in the annular region. The fourth pair of shielding coilsmay be concentric and/or coaxial with the second pair of shielding coilsor third pair of shielding coils.

In some embodiments, the main magnetic body 302 may be a separatedmagnetic body. For example, the main magnetic body 302 may include atleast two separated parts. At least portion of the coils in the mainmagnetic body 302 (e.g., the first shielding coil 402-a, the first coil411-1, the first main magnetic field coils 401-1, the second mainmagnetic field coils 401-2, the third main magnetic field coils 401-3,etc.) may be located in one part, and the rest of the coils in the mainmagnetic body 302 may be located in the other part.

FIG. 5A shows a schematic diagram illustrating exemplary magnetic fieldsproduced by a plurality of main magnetic field coils without a shieldingcoil group according to some embodiments of the present disclosure. Asdescribed in connection with FIG. 4A or FIG. 4B, as shown in FIG. 5A,the plurality of main magnetic field coils 401 (e.g., main magneticfield coils 401-1, main magnetic field coils 401-2, main magnetic fieldcoils 401-3) and the shielding coils 402 (e.g., the first shielding coil402-a, the second shielding coil 402-b) may be coaxially arranged alongan axis (e.g., the axis 405). Merely by way of example, a direction ofthe current within each of the plurality of main magnetic field coils401 may be designated as the first direction the same as illustrated byarrow A, and a direction of the current within each of the shieldingcoils 402 may be designated as the second direction the same asillustrated by arrow B. The first direction is opposite to the seconddirection.

In some embodiments, the main magnetic field B0 produced by theplurality of main magnetic field coils 401 may be represented by dottedbox 501, the magnetic field in a middle region between the plurality ofmain magnetic field coils 401 and the shielding coils 402 (also referredto as a middle magnetic field) may be represented by dotted box 502, andthe magnetic field in an outside region out of the shielding coils 402(also referred to as an outside magnetic field) may be represented bydotted box 503. The middle magnetic field may be a superposition of themagnetic field produced by the plurality of main magnetic field coils401 within the middle region and the magnetic field produced by theshielding coils 402 within the middle region. The outside magnetic fieldmay be a neutralization of the magnetic field produced by the pluralityof main magnetic field coils 401 within the outside region and themagnetic field produced by the shielding coils 402 within the outsideregion. The shade of the color within a dotted box may indicate astrength and distribution of the corresponding magnetic field. As shownin FIG. 5A, the main magnetic field B0 may be the strongest anddistribute uniformly with a direction the same as illustrated by arrowC, the middle magnetic field may be weak and distribute non-uniformlywith a direction the same as illustrated by arrow D, and the outsidemagnetic field may be the weakest with a direction the same asillustrated by arrow C.

FIG. 5B shows a schematic diagram illustrating exemplary magnetic fieldsproduced by a plurality of main magnetic field coils with a shieldingcoil group according to some embodiments of the present disclosure. Asdescribed in connection with FIG. 4B and FIG. 5A, as shown in FIG. 5B,the shielding coil group 411(e.g., the first coils 411-1 and the secondcoils 411-2) may be coaxially arranged with the plurality of mainmagnetic field coils 401 and the shielding coils 402 along an axis(e.g., the axis 405). Merely by way of example, a direction of thecurrent within each of the first coils 411-1 may be the same as thefirst direction illustrated by arrow A, and a direction of the currentwithin each of the second coils 411-2 may be the same as the seconddirection illustrated by arrow B. A magnitude of the current within eachof the first coils 411-1 may be the same as a magnitude of the currentwithin each of the second coils 411-2.

As shown in FIG. 5B, the main magnetic field B0 produced by theplurality of main magnetic field coils 401 may be represented by dottedbox 504, the magnetic field in a middle region between the plurality ofmain magnetic field coils 401 and the second coils 411-2 (also referredto as a middle magnetic field) may be represented by dotted box 505, themagnetic field in the annular region between the first coils 411-1 andthe second coils 411-2 (also referred to as a annular magnetic field)may be represented by dotted box 506, and the magnetic field in anoutside region out of the first coils 411-1 (also referred to as anoutside magnetic field) may be represented by dotted box 507. The shadeof the color within a dotted box may indicate a strength anddistribution of the corresponding magnetic field. In comparison withFIG. 5A, the main magnetic field B0 in FIG. 5B may still be thestrongest and distribute uniformly with a direction the same asillustrated by arrow C, similar to the corresponding magnetic field inFIG. 5A, and the middle magnetic field, the annular magnetic field andthe outside magnetic field in FIG. 5B may all be weaker than thecorresponding magnetic field in FIG. 5A.

FIG. 6A shows a schematic diagram illustrating an exemplary curve of themagnetic field in the annular region without a shielding coil groupaccording to some embodiments of the present disclosure. As shown inFIG. 6A, for a requirement of the main magnetic field B0 to be 1.5 T, amagnitude of the magnetic field in the annular region without theshielding coil group 411 may range from 0.56 T to 0.35 T. A strength ofthe magnetic field in the annular region that is further away from theplurality of main magnetic field coils 401 may be lower than that of themagnetic field in the annular region that is closer to the plurality ofmain magnetic field coils 401.

FIG. 6B shows a schematic diagram illustrating an exemplary curve of themagnetic field in the annular region with a shielding coil groupaccording to some embodiments of the present disclosure. As described inconnection with FIG. 5 and FIG. 6A, a magnitude of the magnetic field inthe annular region with the shielding coil group 411 as shown in FIG. 6Bmay range from 0 to 0.04 T, which is lower than that of the magneticfield in the annular region without the shielding coil group 411 asshown in FIG. 6A. By adding the shielding coil group 411 in the MRIapparatus, one or more components of the radiation therapy apparatusreceived within the annular region may not be influenced by the magneticfield produced by the MRI apparatus as possible.

FIG. 7 shows a schematic diagram illustrating an exemplary curve of themain magnetic field B0 in the MRI apparatus with a shielding coil groupaccording to some embodiments of the present disclosure. As described inconnection with FIG. 5 and FIG. 6 , a magnitude of the magnetic field B0in MRI apparatus with the shielding coil group 411 may keep around 1.5T. By adding the shielding coil group 411 in the MRI apparatus, theplurality of main magnetic field coils 401 of the MRI apparatus may notincrease power consumption, and the magnitude of the main magnetic fieldB0 in the MRI apparatus may not be influenced by the shielding coil 411.

Having thus described the basic concepts, it may be rather apparent tothose skilled in the art after reading this detailed disclosure that theforegoing detailed disclosure is intended to be presented by way ofexample only and is not limiting. Various alterations, improvements, andmodifications may occur and are intended to those skilled in the art,though not expressly stated herein. These alterations, improvements, andmodifications are intended to be suggested by the present disclosure,and are within the spirit and scope of the exemplary embodiments of thepresent disclosure.

Moreover, certain terminology has been used to describe embodiments ofthe present disclosure. For example, the terms “one embodiment,” “anembodiment,” and/or “some embodiments” mean that a particular feature,structure or characteristic described in connection with the embodimentis included in at least one embodiment of the present disclosure.Therefore, it is emphasized and should be appreciated that two or morereferences to “an embodiment” or “one embodiment” or “an alternativeembodiment” in various portions of this specification are notnecessarily all referring to the same embodiment. Furthermore, theparticular features, structures or characteristics may be combined assuitable in one or more embodiments of the present disclosure.

Further, it will be appreciated by one skilled in the art, aspects ofthe present disclosure may be illustrated and described herein in any ofa number of patentable classes or context including any new and usefulprocess, machine, manufacture, or composition of matter, or any new anduseful improvement thereof. Accordingly, aspects of the presentdisclosure may be implemented entirely hardware, entirely software(including firmware, resident software, micro-code, etc.) or combiningsoftware and hardware implementation that may all generally be referredto herein as a “unit,” “module,” or “system.” Furthermore, aspects ofthe present disclosure may take the form of a computer program productembodied in one or more computer readable media having computer readableprogram code embodied thereon.

Furthermore, the recited order of processing elements or sequences, orthe use of numbers, letters, or other designations therefore, is notintended to limit the claimed processes and methods to any order exceptas may be specified in the claims. Although the above disclosurediscusses through various examples what is currently considered to be avariety of useful embodiments of the disclosure, it is to be understoodthat such detail is solely for that purpose, and that the appendedclaims are not limited to the disclosed embodiments, but, on thecontrary, are intended to cover modifications and equivalentarrangements that are within the spirit and scope of the disclosedembodiments. For example, although the implementation of variouscomponents described above may be embodied in a hardware device, it mayalso be implemented as a software only solution, for example, aninstallation on an existing server or mobile device.

Similarly, it should be appreciated that in the foregoing description ofembodiments of the present disclosure, various features are sometimesgrouped together in a single embodiment, figure, or description thereoffor the purpose of streamlining the disclosure aiding in theunderstanding of one or more of the various inventive embodiments. Thismethod of disclosure, however, is not to be interpreted as reflecting anintention that the claimed subject matter requires more features thanare expressly recited in each claim. Rather, inventive embodiments liein less than all features of a single foregoing disclosed embodiment.

In some embodiments, the numbers expressing quantities or propertiesused to describe and claim certain embodiments of the application are tobe understood as being modified in some instances by the term “about,”“approximate,” or “substantially.” For example, “about,” “approximate,”or “substantially” may indicate ±20% variation of the value itdescribes, unless otherwise stated. Accordingly, in some embodiments,the numerical parameters set forth in the written description andattached claims are approximations that may vary depending upon thedesired properties sought to be obtained by a particular embodiment. Insome embodiments, the numerical parameters should be construed in lightof the number of reported significant digits and by applying ordinaryrounding techniques. Notwithstanding that the numerical ranges andparameters setting forth the broad scope of some embodiments of theapplication are approximations, the numerical values set forth in thespecific examples are reported as precisely as practicable.

Each of the patents, patent applications, publications of patentapplications, and other material, such as articles, books,specifications, publications, documents, things, and/or the like,referenced herein is hereby incorporated herein by this reference in itsentirety for all purposes, excepting any prosecution file historyassociated with same, any of same that is inconsistent with or inconflict with the present document, or any of same that may have alimiting affect as to the broadest scope of the claims now or laterassociated with the present document. By way of example, should there beany inconsistency or conflict between the description, definition,and/or the use of a term associated with any of the incorporatedmaterial and that associated with the present document, the description,definition, and/or the use of the term in the present document shallprevail.

In closing, it is to be understood that the embodiments of theapplication disclosed herein are illustrative of the principles of theembodiments of the application. Other modifications that may be employedmay be within the scope of the application. Thus, by way of example, butnot of limitation, alternative configurations of the embodiments of theapplication may be utilized in accordance with the teachings herein.Accordingly, embodiments of the present application are not limited tothat precisely as shown and described.

We claim:
 1. A magnetic resonance imaging (MRI) apparatus configured toacquire MRI data with respect to a region of interest (ROI), the MRIapparatus including: a plurality of main magnetic field coils coaxiallyarranged along an axis; and a plurality of shielding coils arrangedcoaxially along the axis, wherein the plurality of shielding coilsinclude a first coil and a second coil with different sizes.
 2. Theapparatus of claim 1, wherein the plurality of shielding coils areconfigured to reduce influence of a magnetic field in a path of rotationof at least a portion of a radiation source of the radiation therapyapparatus.
 3. The apparatus of claim 1, wherein a direction of a currentwithin the first coil is opposite to a direction of a current within thesecond coil.
 4. The apparatus of claim 1, wherein a radius of the firstcoil is greater than a radius of the second coil which is concentricwith the first coil.
 5. The apparatus of claim 1, wherein a direction ofa current within the first coil is the same as a direction of a currentwithin the main magnetic field coils.
 6. The apparatus of claim 1,wherein a radius of the first coil or the second coil is larger thanthat of the plurality of main magnetic field coils.
 7. A therapeuticapparatus including: a magnetic resonance imaging apparatus configuredto acquire magnetic resonance imaging data with respect to a region ofinterest, wherein the magnetic resonance imaging apparatus includes anannular cryostat in which a plurality of main magnetic field coils and aplurality of shielding coils are arranged coaxially along an axis of theannular cryostat; and a radiation therapy apparatus, wherein theradiation therapy apparatus includes a radiation source for directingtherapeutic radiation to at least one portion of the region of interest,wherein the annular cryostat includes a recess at an outer wall, and atleast a portion of the radiation source is within the recess, and atleast one of the shielding coils is configured to reduce and/or shieldinfluence of the magnetic field on a region within the recess.
 8. Theapparatus of claim 7, wherein at least one shieling coil of theshielding coils is arranged close to a bottom of the recess, and acurrent within the at least one shieling coil of the shielding coils isin an opposite direction with a current within the main magnetic fieldcoils.
 9. The apparatus of claim 7, wherein at least one shieling coilof the shielding coils is arranged close to an opening of the recess,and a current within the at least one shieling coil of the shieldingcoils is in the same direction with a current within the main magneticfield coils.
 10. The apparatus of claim 7, wherein the shielding coilsinclude a first coil and a second coil with different sizes.
 11. Theapparatus of claim 10, wherein a direction of a current within the firstcoil is opposite to a direction of a current within the second coil. 12.The apparatus of claim 11, wherein a radius of the first coil is greaterthan a radius of the second coil which is concentric with the firstcoil.
 13. The apparatus of claim 12, wherein the direction of a currentwithin the first coil is the same as a direction of a current within themain magnetic field coils.
 14. The apparatus of claim 10, wherein atleast portion of the annular recess is located radially between thefirst coil and the second coil.
 15. A magnetic resonance imaging (MRI)apparatus configured to acquire MRI data with respect to a region ofinterest (ROI), the MRI apparatus including: an annular cryostat; aplurality of main magnetic field coils coaxially arranged along an axisof the annular cryostat; at least a first pair of shielding coilsconfigured to reduce and/or shield influence of a magnetic field outsidethe MRI apparatus, and a direction of a current within the first pair ofshielding coils is opposite to a direction of a current within the mainmagnetic field coils.
 16. The apparatus of claim 15, wherein the MRIapparatus further includes a second pair of shielding coils, the firstpair of shielding coils and the second pair of shielding coils are withdifferent sizes, and the second pair of shielding coils is configured toreduce and/or shield influence of a magnetic field between the firstpair of shielding coils and the main magnetic field coils.
 17. Theapparatus of claim 16, wherein a direction of a current within thesecond pair of shielding coils is the same as the direction of a currentwithin the main magnetic field coils, the second pair of shielding coilsbeing close to the first pair of shielding coils.
 18. The apparatus ofclaim 16, wherein a direction of a current within the second pair ofshielding coils is opposite to the direction of a current within themain magnetic field coils, the second pair of shielding coils beingclose to the main magnetic field coils.
 19. The apparatus of claim 16,wherein the apparatus further includes a third pair of shielding coils,a shielding coil of the third pair of shielding coils being concentricwith a shielding coil of the second pair of shielding coils.
 20. Theapparatus of claim 16, wherein a direction of a current within the thirdpair of shielding coils is opposite to a direction of a current withinthe second pair of shielding coils.